Another Larry Brasfield Self-Aggrandizing Put-Down Anecdote

That's sort of what I've been trying to tell Larry all along. Different equations for different things don't have to have the same type of curve.

Actually, the flaw was the assumption that a linearly varying SI unit should dictate a linearly varying derived unit. It's like saying that if the diameter increases linearly, so should the area.

Amen, bro. Maybe it was intuition, but I bitched before doing the math becasue the flawed logic was all too apparent to me. I just tried a different approach for grins.

LOL

Not true. You can determine that the two aren't equal for all t. That is all that is really required, and is the source of the puzzle. When I said 'answers', I obviously meant energy of rocket at time t, and energy expended in fuel up till time t. Sorry for any confusion.

The original "puzzle" was a

Except for the last part, I agree. However, that doesn't make it a bad puzzler. Some of those old martin gardner puzzlers were quite devious. Riddles are notoriously devious, and mislead you in whatever way they can. Mysteries are fun for that very reason.

It was similar to the 'Questions' section at the end of chapters in my first college physics book. They were generally qualitative puzzlers like this, that were intended to make you think about what could be happening.

Here is an example:

"A canoeist in a still pond can reach shore by jerking sharply on the rope attached to the bow of the canoe. How do you explain this? (yes she can! - it's true)" (question 11, chap 9, "Physics, Part I", Halliday and Resnick, 3rd edition).

Sadly, I don't know the answer to this, other than some hand wavings about friction of the water on the boat. The chapter is entitled "Conservation of Linear Momentum", so it probably has something to do with conservation of momentum... ;) Maybe somebody with a few more intact brain cells left over from college can explain it.

My issue (and Fred's, I'm gussing) wasn't really about the puzzler itself, it was with the "Let's see if we can show this guy up" attitude it was originally posted with. However, I like these kinds of things; they are more fun than crossword puzzles. Of course, I knew that it had to be the fuel, but I wanted to come up with an equation the proved that KE(fuel) + KE(rocket) = PE expended for all time (until the thing runs out of fuel). He thus had me digging out old physics text books. It was fun, even though I didn't succeed in coming up with the equation.

Now you're becoming a bore ;) really - Read my posts *and* the OP. I've been saying something close to that all along, to wit: KE is proportional to the square of velocity which is proportional to acceleration which is proportional to force or thrust. You don't have to ponder/know/guess the conversion of fuel to thrust or the efficiency to see that. At least I don't, nor others here that are most likely sitting back laughing at this bullsh*t.

I said it better with my math - once with conservation of momentum principles and once with conservation of energy principles. Thanks for writing out your thoughts with equations, but as you said, the non-puzzle was why a linearly varying quantity (mass) does not produce another linear varying quantity (energy)

To say that v^2 is proportional to 3*KE is nothing more than plugging in numbers. It kind of obscured the real problem with the non-puzzle, at least for me. And all those words.

Thanks again.

I solved every single problem in that book some 100 years ago when I took my first course, but don't remember that one. However, the "jerk" leads me too suspect you might want to look at what they define as "impact" which imparts a step momentum to the canoe which carries it to shore.

You can do it- forget digging out equations- use your understanding of momentum and common sense.

That's the book I had in HS and used at U of PA. I bought it when I moved to a new school. And that's a damned good question. Thanks, I'll remember to look at those noodle twisters when I'm bored.

I think it's because the energy she uses to jerk her hand is internal to her [strike lewd remark] body. If she stopped the motion on her own, an oppositely directed energy from inside her body would negate that. But the rope stops her motion and that is external to her body.

I don't know for sure that I ever had any brain cells. I though this was all an illusion.

Yup and yup.

Free your mind Neo ;)

I read in sci.electronics.design that Robert Monsen wrote (in ) about 'Another Larry Brasfield Self-Aggrandizing Put-Down Anecdote', on Thu, 24 Mar 2005:

Some of the PE is the chemical energy in the unburned fuel. It's also necessary to know how much of that is released by combustion.

I think you can work with a constant momentum of close system with no external force applied- so that M(t)*dV/dt=-V*dM(t)/dt is just another way of saying the rocket acceleration is proportional to mass rate of burn of fuel, and also that a linear time rate of change in velocity is produced by the linear time rate of change of fuel mass, the requirement of constant thrust.

It has to do with F=M*dV/dt which for small time intervals (jerk) becomes F*dt=(Vf-Vi) or in this simple case, Vf(velocity imparted to canoe)=F*dt, where F is impulse force she applied to rope during the jerk of duration dt- so many pounds*milliseconds.

This must be a clue:

I got the gist after a few paras. I don't know in what context you used the phrase, but I'll venture that Larry meant that his teacher just kept on until he gave in.

No. I did it twice with math and with too many attempts at words. I'm going to take the salient advice of you and some others, if I can control myself :) If I can't point out the flawed logic with words on the first try, I'll just do and show the math if so inclined.

Getting that in ascii was a PITA. Maybe mathematica to html would be better.

Damn Fred. Right after I posted that, I went and had some sardines and was thinking that if I were to try to quatify that, I'd look at impulse and momentum. The key is that her jerk/motion energy is internal to her and the stopping energy is imparted by the canoe.

I am assuming that 1) the rope is tied off to a pier, and 2) she remains fixed to the canoe, but in the case of some women you can't be too sure about that:-)

If you look at it that way, it is puzzling. If the fuel burn process results in constant thrust, Fthrust, then the power this imparts to the rocket is Fthrust*Vrocket. This should tell you that fuel burn does NOT apply energy to the rocket at constant rate, it increases with increasing velocity.

There's nothing to it. They even did it with the Enterprise once when they were stuck in some kind of negative energy vortex thing. They hit the thrusters with one pulse, and coasted out. It's the same thing with the canoe. The energy of the jerk is imparted to the canoeist/canoe system, and the canoe simply coasts to shore. Imagine you're standing on the dock, holding the rope, which is attached to the canoe. Could you jerk that rope sharply and have the canoe come to you? Of course. Same thing. I was stuck on a raft in the middle of a pond without a paddle once, and didn't want to get wet because Mom would kill me for playing around in the pond (actually a backed-up drainage ditch), so I kind of rocked it asymmetrically, turning the whole raft into sort of a paddle. I made it back to shore.

What I wonder about is the assertion that a rocket's speed, relative to some imaginary fixed frame of reference, can't ever be faster than the speed of the exhaust relative to the rocket. I wonder if that's really true?

Thanks, Rich

He posted another, challenging us to explain why a transistor's Ic goes down when you zener the b-e junction. This one also has a trick answer: it doesn't.

John

Better clean that up. My fumbling fingers typed "=" instead of "-" in two places.

let D be the round d

k = fuel mass expenditure wrt time M = initial mass of rocket

m = M - kt instantaneous mass of rocket

expand that DE 1 2

-- = - v Dm 2

DE 2

-- = mat - kat Dv

dE 1 2 dm dv 2 dv

-- = - v -- + mv -- - kat -- dt 2 dt dt dt dE 1 2 2

-- = - kv + mav - kv dt 2

integrate 1 2 2 E = - kv t + Fvt - kv t 2

. . . Total energy = KE + thrust energy - exhaust energy

QED

Ok. I suck at ascii math and my work on paper was sloppy. Why don't we just build this rocket car thing already?

let D be the round d

k = fuel mass expenditure wrt time dm/dt or m/t M = initial mass of rocket

m = M - kt instantaneous mass of rocket

expand that and take the partial derivatives wrt m and v DE 1 2

-- = - v DM 2

DE 2

-- = Mat - kat Dv

dE 1 2 dM dv 2 dv

-- = - v -- + Mv -- - kat -- dt 2 dt dt dt dE 1 2 2

-- = - kv + Mav - kv dt 2

integrate

. . . Total energy = KE + thrust energy - exhaust energy

QED

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

That whole line of thinking is insulting and founded only on misreading between the lines. How anybody can take it as more than a mystified kid asking the question because he wants an answer says more about the conjecturers as it does about my motive.

You have misconstrued what I said.

I wrote "One 2nd order effect is that the extra majority carriers reduce the equilibrium density of minority carriers in the base region, reducing what is usually thought of as the C-B leakage."

I never claimed it was the only 2nd order effect, and explicitly said it was not: "I'm only stating the effect of the 2nd order effect I mentioned. There are a few others. You have to add them all up if you're nuts enough to want to predict how they will show up in real devices."

And the posed question said nothing about Ic going down. On that, it said only "What happens to the current as indicated by the meter? Now, why?

You have already seen that the effects are small, being 4+ orders of magnitude down. Why do you insist that what you see is contrary to what I've stated? I only claimed that there would be small effects. The fact that I named one whose sign differs from what you saw does not contradict the thrust of my prediction, which is that there is no 1st order (ie "large") effect.

And to deal with another matter, there is no "trick" answer. There are answers founded on facts and reasoning, and all the rest.

Socratic method is a teaching scheme which uses questions to prompt students toward a particular answer, rather than just telling them what to think. The basic idea it was based on was that you already know everything (in the 'ideal plane'), but have forgotten some of it due to your existence here in the real world. The questions simply prompt you to remember what you've forgotten. It's annoying, and doesn't really work all that well, but educators seem to like it because it gives them a sense of superiority. They get to watch the student squirm while trying to come up with an answer the teacher wants.